Radial tires for motor vehicles are generally known to present, along the inner periphery, two metal beads, each fitted, along the outer periphery, with an elastomeric filler usually in the form of a substantially triangular section wing. Each bead and respective filler are covered by a respective lateral portion of a body ply, the end portion of which is joined to the outer surface of a center portion of the body ply over the outer end of the filler. The inner and outer surfaces of the package so formed are usually covered with an innerliner and abrasion strip respectively. The innerliner and abrasion strip are integral with each other, usually of substantially constant thickness, and turned about a respective lateral portion of the body ply.
These fillers were not usually featured on diagonal tires, the required lateral rigidity of which was provided for by a number of diagonal cross plies. As these are absent on radial tires, however, the fillers have always been considered essential for ensuring sufficient lateral rigidity. The fillers which even today are considered indispensable by radial tire manufacturers, involve a number of drawbacks both in terms of design and cost.
First, a preassembly line parallel with the tire assembly line is required for forming and preassembling the metal beads and fillers, which are then fed on to the tire assembly line. In addition to the outlay involved for preassembling the beads and fillers, such a system rules out practically any possibility of the metal beads being formed automatically during tire assembly, and involves the use of complex equipment for feeding the bead-filler packages on to the tire building drum.
Second, the fillers require the use of extremely wide body plies, each turned-up lateral portion of which must be long enough to cover the surface of the filler facing outwards of the tire, and also join up with the outer surface of the center portion of the body ply.
Finally, the presence of the fillers, the mix of which obviously differs from that of the surrounding parts, may result in a poorly homogeneous bead area and, consequently, may cause curing problems. Despite the rubber industry now being in a position to overcome problems of this sort, even the slightest variation in the mix of any one of the components in the bead area of a radial tire inevitably means varying the mix of all the component parts involved, for ensuring curing is performed as homogeneously as possible.
Notwithstanding the above precautions, however, the bead areas of radial tires are still considered "critical" by manufacturers, due to the severe stress they are subjected to, and the presence of a number of superimposed nonhomogenous layers bonded together.